1. Field of the Invention
The present invention is related to capacitive sensing touch screen systems, and, more particularly, to a touch screen system and related robust method of operation in which finger noise is rejected and the most accurate touch information possible is extracted.
In many electronic applications, a physical parameter can be monitored by the variation of a given capacitance. This is the case for pressure sensors, movement sensors, accelerometers, as well as other sensor circuits. In the case of projective capacitive touch technology, the capacitance to be measured is accessible through both terminals of a capacitor.
A projective capacitive touch matrix 100 is shown in FIG. 1 and described below. A portion 104 of the actual touch screen 102 is shown including a plurality of capacitors accessed by the R-lines (Rows) and C-lines (Columns). A finger 106 that is pressed against the touch screen portion 104 implicates four fringing capacitances 108. The perturbation by finger 106 can be sensed on the C-lines and the R-lines. The fringing capacitances exist on their own without the perturbation of a touching finger. The presence of the touching finger modifies the value of the fringing capacitances.
A touch screen is commonly a “transparent touch matrix”, which is stacked on top of a display means. This is referred to as “on-cell” technology. The touch matrix can be also “merged” within the display mean and this is referred to as “in-cell” technology. In the example of FIG. 1, the touch matrix is a one layer ITO diamond type. Other types of touch screen systems are known in the art.
A touch device such as a touch screen is a means for detecting whether something (such as a finger, nail, pen, or anything else) is touching (touch detection) or approaching (proximity detection) the touch screen. The touch device must also detect the location of the touch event. The reading of the touch event is accomplished by measuring the Ci and Rj capacitance (Cs) variations. Capacitance Cs is composed of four fringing capacitances as is shown in further detail in FIG. 2. A portion 204 of the touch screen is shown showing the four fringing capacitances 208, the capacitance CFINGER of the finger 206, as well as the noise Vnpp associated with noise from a user's body.
There are many different ways to measure the Cs capacitance variations, but they are all based on the same principle. Predetermined known electrical quantities (voltage, current, charges . . . ) are forced into the touch matrix and, in response, certain modulated electrical values are read out from the matrix. The presence of a finger (as well as a nail, pen, or other item that may be used in conjunction with a touch screen) locally affects the electrical field that is created by the “forced electrical quantity” and the modulation of the value of the fringing capacitances and is detected as a touch event. The contact between the finger and the touch matrix can be assumed to be a capacitive contact (CFINGER as shown in FIG. 2), and the electrical noise present in the user's body is injected inside the touch matrix.
In all touch screen systems designs, what is desired is the ability to reject the finger noise and the ability to extract the most accurate touch information. The ability to sense whether in fact there is a touch or not and the strength and location of that touch is the main challenge in designing a capacitive sensing analog front end.
2. Relevant Background
Capacitive touch sensor technology is widely used in mobile, computing, and even in consumer applications. The working principle is that the touch screen capacitance is decreased when an object, i.e., a finger, is moving closer to a capacitor. The capacitance changes are detected by a sensor circuit to indicate existence of the object. The capacitance value is in the range of 1 pF to 5 pF and the capacitance change is about 10%. Hence detection of the capacitance value is very sensitive to any noise introduced in the system.
To be more precise, the capacitance decreases in the case of projective capacitance technology where a mutual capacitance is measured. However, some other technologies measure a self-capacitance (relative to ground) and, in these cases, the capacitance actually increases when a touch occurs.
There are various forms of capacitive touch sensor architectures known in the prior art. Several examples are listed below:
United States Patent Application Publication No. 2009/0244014 teaches a method using a charge amplifier (see FIG. 3A therein);
United States Patent Application Publication No. 2010/0097077 teaches a method using charge transfer and a clock period counter (see FIG. 3 therein);
United States Patent Application Publication No. 2008/0007534 teaches a method using a relaxation oscillator and a digital counter (see FIG. 3B therein);
U.S. Pat. No. 5,854,625 teaches a method using an oscillator (see FIG. 5 therein); and
United States Patent Application Publication No. 2009/0322410 teaches a method using a charging period comparison (see FIG. 6 therein).